Indirect evaporative cooler using membrane-contained liquid desiccant for dehumidification and flocked surfaces to provide coolant flow

ABSTRACT

An apparatus for conditioning an inlet air stream. A first stage is provided with a dehumidifier cooling an air stream input by absorption of water vapor from the input air stream. A second stage is provided with an indirect evaporative cooler to receive a cooled portion of the input air stream and sensibly cool the received portion of the input air stream to a temperature range near the dew point temperature. A first portion of the sensibly cooled air stream is exhausted to a cooled space while a second portion is directed to a wet side of the indirect evaporative cooler and receives heat to sensibly cool the input air stream. A flow channel for the second portion of the sensibly cooled air stream in the indirect evaporative cooler is defined by a surface of a separation wall covered with wicking material acting to wick a stream of liquid coolant.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/662,146, filed Jun. 20, 2012, which is incorporated herein in itsentirety.

CONTRACTUAL ORIGIN

The United States Government has rights in this invention under ContractNo. DE-AC36-08G028308 between the United States Department of Energy andthe Alliance for Sustainable Energy, LLC, the Manager and Operator ofthe National Renewable Energy Laboratory.

BACKGROUND

Air conditioning is used worldwide to provide comfortable and healthyindoor environments that are properly ventilated and cooled and thathave adequate humidity control. While being useful for conditioningsupply air, conventional air conditioning systems are costly to operateas they use large amounts of energy (e.g., electricity). With thegrowing demand for energy, the cost of air conditioning is expected toincrease, and there is a growing demand for more efficient airconditioning methods and technologies. Additionally, there areincreasing demands for cooling technologies that do not use chemicalsand materials, such as many conventional refrigerants, that may damagethe environment if released or leaked. Maintenance is also a concernwith many air conditioning technologies, and, as a result, any newtechnology that is perceived as having increased maintenancerequirements, especially for residential use, will be resisted by themarketplace.

Evaporative coolers are used in some cases to address air conditioningdemands or needs, but, due to a number of limitations, conventionalevaporative coolers have not been widely adopted for use in commercialor residential buildings. Evaporative coolers, which are often calledswamp coolers, are devices that use simple evaporation of water in airto provide cooling in contrast to conventional air conditioners that userefrigeration or absorption devices using the vapor-compression orabsorption refrigeration cycles. The use of evaporative cooling hastypically been limited to climates where the air is hot and humidity islow such as in the western United States. In such dry climates, theinstallation and operating costs of a conventional evaporative coolercan be lower than refrigerative air conditioning. Residential andindustrial evaporative coolers typically use direct evaporative coolingwith warm dry air being mixed with water to change the water to vaporand using the latent heat of evaporation to create cool moist air (e.g.,cool air with a relative humidity of 50 to 70 percent). For example, theevaporative cooler may be provided in an enclosed metal or plastic boxwith vented sides containing a fan or blower, an electric motor tooperate the fan, and a water pump to wet evaporative cooling pads. Toprovide cooling, the fan draws ambient air through vents on the unit'ssides and through the dampened pads. Heat in the air evaporates waterfrom the pads, which are continually moistened to continue the coolingprocess. The cooled, moist air is then delivered to the building via avent in the roof or a wall.

While having an operation cost of about one fourth of refrigerated airconditioning, evaporative coolers have not been widely used to addressneeds for higher efficiency and lower cost conditioning technologies.One problem with many sump coolers is that in certain conditions theseevaporative coolers cannot operate to provide adequately cooled air. Forexample, air may only be cooled to about 75° F. when the input air is90° F. and 50 percent relative humidity, and such cooling may not beadequate to cool a particular space. The problem may get worse astemperatures increase well over 100° F. as found in many locations inthe southwest portion of the United States and elsewhere. As a result,the air conditioning system may need to include refrigerated airconditioning to cool the outlet air from the evaporative cooler, whichresults in a system that is more expensive to purchase, operate, andmaintain.

Additionally, conventional evaporative coolers provide nodehumidification of the air and, in fact, often output air at 80 to 90percent relative humidity, which may only be acceptable in very dryenvironments as very humid air reduces the rate of evaporation foroccupants of the building (e.g., reduces comfort levels) and can causecondensation resulting in corrosion or other problems. Dehumidificationis provided as a second or later stage in some evaporative coolers suchas by wicking a liquid desiccant along a wall of the air flow channel orchamber, but such systems have not been widely adopted due to increasedoperating and maintenance costs and concerns of having the desiccantexpelled with the conditioned air. In general, maintenance is a concernwith evaporative coolers as the evaporation process can result inmineral deposits on the cooling pads and other surfaces of the coolerthat need to be cleaned or replaced to maintain the efficiency of thesystem, and the water supply line needs to be protected against freezingduring the off season such as by draining the system. Due to these andother concerns, evaporative cooling is unlikely to be widely used toprovide an energy efficient, air conditioning alternative for commercialand residential applications until significant improvements are madethat address maintenance concerns while improving achievable cooling(e.g., providing adequately cooled output air for direct use in abuilding).

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods that aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

This is achieved, in part in some applications, by providing a mass/heattransfer assembly for use in indirect evaporative coolers or heatexchangers. The assembly is formed of alternating stacks each includinga first (or upper) layer or sheet of membrane material, a separationwall, and a second (or lower) layer or sheet of membrane material. Themembrane or membrane material for each layer is permeable to watermolecules in the vapor state while the separation wall is impermeable towater but allows heat transfer (e.g., is a thin layer and/or is made ofmaterials that conduct heat). In a first one of adjacent pairs ofstacks, coolant such as water flows between the first membrane layer andthe separation wall and liquid desiccant flows between the separationwall and the second membrane layer while in the second or next one ofthe adjacent pairs of stacks the flow order is reversed. This orderingis repeated throughout the mass/heat transfer assembly to formalternating supply and exhaust air flow channels or chambers. Supply air(or air to be conditioned) is directed through a channel between a firstpair of stacks while a portion of the pre-cooled exhaust air (e.g., afraction of the supply air that is cooled by flowing through the stacks)is directed through a chamber between a second or next pair of stacks(e.g., typically in a counterflow arrangement relative to the flow ofthe incoming supply air). Liquid desiccant is provided proximate to thesupply inlet airflow while coolant such as water is provided proximateto the exhaust airflow (i.e., a fraction of supply outlet airflowdirected to be exhausted) with the air only being separated from theseflowing liquids by the water permeable membrane. The supply air inletairflow, supply outlet airflow, exhaust airflow, liquid desiccant flow,and coolant flow are plumbed such as via one or more manifold assembliesto the mass/heat transfer assembly, which can be provided in a housingas a single unit (e.g., an indirect evaporative cooler).

In a typical embodiment, dehumidification and evaporative cooling areaccomplished by separation of the air to be processed and the liquidand/or gas substances (e.g., liquid desiccant, water, desiccated air,and the like) by a membrane. The membrane is formed of one or moresubstances or materials to be permeable to water molecules in the vaporstate. The permeation of the water molecules through the membrane is adriving force behind (or enables) dehumidification (or dehumidificationin some implementations) and evaporative cooling of one or more processair streams. As described above, multiple air streams can be arranged toflow through chambers in the mass/heat transfer assembly such that asecondary (purge) air stream, such as the exhaust airflow of pre-cooledsupply air, is humidified and absorbs enthalpy from a primary (process)air stream, such as the supply inlet airflow that can then be directedto a building as supply outlet airflow (e.g., make up air for aresidential or commercial buildings or the like). The process air streamis sensibly cooled and is, in some embodiments, simultaneouslydehumidified by providing a liquid desiccant flow contained by membranesdefining the sidewalls of the supply inlet airflow channel or chamber.

The membrane is also used in some embodiments to define sidewalls of theexhaust (e.g., counter) airflow channel or chamber such that themembrane controls or separates coolant liquid from the exhaust airstream. Wicking materials/surfaces or other devices may be used tocontain or control water flow (e.g., direct-contact wicking surfacescould be used in combination with the use of the liquid desiccantcontainment by a membrane), but membrane liquid control facilitatesfabrication of the stacks or manifold structure useful for heat and massexchanger/assembly configurations described herein that provide cooling,dehumidification, and/or humidification. In such configurations, the airstreams can be arranged in counter-flow, counter-flow with pre-cooledexhaust air, cross-flow, parallel flow, and impinging flow to performdesired simultaneous heat and mass transfer in the evaporative coolingunits.

By way of example, but not limitation, an embodiment includes anindirect evaporative cooler for cooling a stream of inlet supply airfrom a first temperature to a second, lower temperature using a streamof liquid coolant and a stream of exhaust or purge air. The coolerincludes a first flow channel through which the stream of inlet supplyair flows and a second flow channel adjacent the first flow channelthrough which the stream of exhaust air, at a lower temperature than theinlet or first temperature of the supply air, flows. The second flowchannel is formed or defined in part by a sheet of a membrane ormembrane material that is permeable to water vapor but that otherwisecontains the liquid coolant. In this manner, the coolant flows on a sideof the membrane (and not in direct contact with) the air in the secondflow channel but mass is transferred as a vapor through the membrane tothe exhaust air when or in response to heat being transferred from theinlet supply air to the liquid coolant. In some cases or configurations,as will become clear, the supply air stream (or inlet supply air) iscooled and dehumidified in this first stage. A second stage may beprovided to sensibly cool the air stream to a very cool temperature,which could be below the dewpoint of the original supply inlet air as itwas dehumidified initially or in the first state to allow this.

A separation wall that is spaced apart from the sheet of membrane isused to define a flow channel for the liquid coolant, with the wallbeing formed from a material (such as plastic) that is impermeable tothe liquid coolant but that conducts or allows the heat to betransferred from the inlet air supply to the coolant. A second sheet ofmembrane may be spaced apart from the opposite side of this separationwall to define a flow channel for a liquid desiccant, and duringoperation, water vapor is transferred from the stream of inlet supplyair through the membrane to the liquid desiccant, which results in theinlet supply air being concurrently cooled and dehumidified. Themembrane is effective for resisting or even fully blocking flow of theliquid coolant and the liquid desiccant while allowing flow of watervapor, and, in some embodiments, the coolant is water and the desiccantis a halide salt solution (e.g., a weak desiccant such as CaCl or thelike). The exhaust air in some cases is a redirected portion of thestream of inlet supply air after it has been cooled to the second, lowertemperature (e.g., as it is exiting the first flow channel), and theexhaust air may flow in a direction through the second flow channel thatis cross, counter, or a combination of these relative to the supply airflowing in the first flow channel.

In another exemplary embodiment, a method is provided for conditioning aprocess or return air for a residential or commercial building. Themethod includes first directing the process air through a first flowchannel and second directing a stream or volume of liquid desiccantadjacent one or more walls defining the first flow channel, the liquiddesiccant is separated from the process air by a membrane (e.g., themembrane provides the walls) that contains the liquid desiccant and alsoallows water vapor from the process air to flow into and be absorbed bythe liquid desiccant, which dehumidifies the process air. The methodfurther includes concurrent with the first and second directing, thirddirecting a stream of purge air through a second flow channel proximateto the first flow channel (e.g., parallel and adjacent). The purge airis at a temperature lower than all or at least a substantial portion ofthe process air in the first flow channel, and in some cases, the purgeair is a fraction of the dehumidified process air exiting the first flowchannel that is directed in a counter flow direction relative to theprocess air through the second flow channel. The method also includesfourth directing a stream of liquid coolant adjacent a wall of thesecond flow channel. The liquid coolant is also separated from the airby a membrane that is permeable to vapor from the coolant such that massis transferred from the coolant to the purge air. The method providesfor concurrent (or single stage) dehumidification and cooling of theprocess air.

According to another aspect, a mass and heat transfer assembly isprovided for use in an indirect evaporative cooler or exchanger device.The assembly includes a first stack including an upper membrane, a lowermembrane, and a separation wall between the upper and lower membranes.The upper and lower membranes are permeable to water in vapor form andthe separation wall is substantially impermeable to liquid and vapor.Second and third stacks are provided that also each includes an uppermembrane, a lower membrane, and a separation wall positionedtherebetween. In the assembly, the first stack and second stacks arespaced apart (such as less than about 0.25 to 0.5 inches apart) todefine a flow channel for receiving a first stream of air (e.g., air tobe conditioned) and the second and third stacks are spaced apart todefine a flow channel for a second stream of air (e.g., purge or exhaustair directed in cross or counter flow relative to the first stream ofair). In some configurations and/or operating modes, the device doesonly evaporative cooling and no dehumidification. Such that themembranes are only used on the purge side and the other side of the wallis left bare for the supply air to exchange heat.

The first, second, and third stacks may be considered a set of stacks,and the assembly includes a plurality of such sets of stacks to define aplurality of air flow channels spaced apart by the stacks or layers ofmembranes and separation walls. A divider or separator may be providedin the flow channels to maintain spacing of the membranes while allowingflow of the air streams in the channels. The assembly may furtherinclude in the first stack a liquid coolant flowing between the uppermembrane and the separation wall and a liquid desiccant flowing betweenthe separation wall and the lower membrane. In the second stack, aliquid desiccant flows between the upper membrane and the separationwall while a liquid coolant flows between the separation wall and thelower membrane. In the third stack, liquid desiccant flows between theupper membrane and the separation wall while liquid coolant flowsbetween the separation wall and the lower membrane. The liquid coolantmay be water and during operation water vapor may be transferred fromthe coolant through the membrane to the second stream of air. The liquiddesiccant may be a salt solution (such as weak desiccant such as CaCl,or the like) and during operation or use of the assembly water vapor maybe transferred from the first stream of air through the membrane to theliquid desiccant, whereby the first stream of air is simultaneouslydehumidified and cooled to a lower temperature.

In some cases, the mass and heat transfer assembly may be configuredwith no membrane on the coolant (e.g., water) side of the device. Insuch a mass transfer assembly, liquid desiccant is contained by a vaporpermeable membrane in the combined stacks, as discussed above. However,the coolant, which in many cases is water, is allowed to flow withoutmembrane containment. To this end, the coolant is maintained or attachedon a surface or side of the separator or separation wall through the useof surface tension forces on a wicked or flocked surface (e.g., awicking layer is attached to the separation wall surface). The flockedsurface or layer of wicking material is attached to the separation wall,and, thus, there is direct thermal contact between the separation walland the liquid coolant (e.g., water flowing through the wickingmaterial). Water evaporation occurs freely between thiscoolant-soaked/containing surface on the separation wall and the purgeor exhaust air stream.

Further, the mass and heat transfer assembly may include ahumidification stage. The heat and mass transfer assembly may include anassembly or section where water adjacent to a supply air stream ismembrane contained with a vapor permeable membrane or in a layer ofwicking material. The supply air would be in contact with the membraneand allow for humidification of the supply air stream prior to dischargefrom the mass and heat transfer assembly (e.g., a humidifier stageprovided downstream from the sensible or indirect evaporative coolerstage).

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than limiting.

FIG. 1 illustrates in schematic form an evaporative cooler or heatexchanger including an exemplary representative of a permeable membranestack or assembly for use in providing indirect evaporative coolingconcurrently with dehumidification in an integral unit or single stage;

FIG. 2 illustrates another exemplary representation of an evaporativecooler showing an assembly of membrane/wall/membrane stacks used incombination to direct the supply and exhaust airflows relative tomembrane-contained liquid desiccant and coolant (e.g., cooling water) toachieve cooling and dehumidification;

FIG. 3 illustrates an evaporative cooler similar to that shown in FIG. 2but being configured with integral counterflow passages forexhaust/cooled air;

FIG. 4 is a top view of an exemplary heat exchanger illustrating airflows through a plurality of channels or chambers provided bymembrane-based assemblies such as those shown in FIGS. 1-3 or otherembodiments shown or described herein;

FIG. 5 (specifically, 5A and 5B) illustrates an exemplary modeling of anevaporative cooler or counterflow heat/mass exchanger such as one withthe stack assembly shown in FIG. 2 and flow arrangement shown in FIG. 4;

FIG. 6 is a graph of air flow and surface temperatures along the lengthof the exchanger modeled as shown in FIG. 5;

FIG. 7 is a graph of humidity ratios of the air along the length of theexchanger modeled as shown in FIG. 5;

FIG. 8 is a graph showing concentration of liquid desiccant flowingthrough the modeled heat exchanger of FIG. 5;

FIG. 9 is a psychrometric chart showing the cooling and dehumidifyingprocess modeled as shown in FIG. 5;

FIG. 10 is a top view of another exemplary heat exchanger illustratingair flows through a plurality of channels or chambers provided bymembrane-based assemblies such as those shown in FIGS. 1-3, or otherembodiments shown or described herein;

FIG. 11 is a top view of another exemplary heat exchanger similar tothose shown in FIGS. 4 and 10 showing a differing unit arrangement withdiffering exhaust airflows;

FIG. 12 is a psychrometric chart showing the cooling and dehumidifyingprocess modeled similar to the modeling shown in FIG. 5 for theconfiguration of a heat exchanger shown in FIG. 10;

FIG. 13 illustrates a HVAC system using an indirect evaporative coolerto provide conditioned air to a building;

FIG. 14 is a psychrometric chart providing results of one test of aprototype fabricated similar to the embodiment of FIG. 4 with the stackassembly of FIG. 2;

FIG. 15 illustrates in schematic form an evaporative cooler or heatexchanger similar to that shown in FIG. 1 including anotherrepresentative permeable membrane stack or assembly;

FIGS. 16 and 17 illustrate in schematic form two humidification sections(or portions of a stack that may be provided in such a section), each ofwhich makes use of a wicking layer wetted with water or otherhumidification fluids/sources;

FIGS. 18, 19, and 20 provide, respectively, a schematic side view of atwo-stage evaporative cooler, a top view of a pair of first and secondstage stacks used to form the cooler, and a psychrometric chart of thecooling process provided during operation of the two-stage evaporativecooler; and

FIGS. 21 and 22 provide, respectively, a top view of a cooler similar tothat of FIG. 19 but with an added direct evaporative stage and apsychrometric chart of the cooling process during operation of thecooler.

DETAILED DESCRIPTION

The following provides a description of exemplary indirect evaporativecoolers with dehumidification and mass/heat transfer assemblies for suchcoolers that provide inlet air stream chambers with sidewalls defined bypermeable membrane sheets containing liquid desiccant. The assembliesalso include outlet or exhaust air stream chambers (such as incounterflow to the inlet air streams) with sidewalls defined bypermeable membrane sheets containing coolant such as water. Inembodiments described below, the membrane is “permeable” in the sensethat moisture in the form of a vapor (e.g., water in the vapor state)generally can permeate readily through the membrane such as from aninlet supply air and from liquid coolant via evaporation. However, themembrane generally contains or blocks moisture in the form of a liquidfrom flowing through as it is instead directed to flow within thechannel or chamber. In some cases, water in the liquid state iscontained by the membrane at pressures less than about 20 psi and moretypically less than about 5 psi. The coolant and the liquid desiccant insome embodiments are maintained at pressures below about 2 psi, and thepermeable membrane contains moisture such as water in the liquid statewhile water vapor permeates the membrane.

As will become clear from the following description, use of theassemblies such as for evaporative coolers or mass/heat exchangersprovides a number of benefits. The inlet or process air stream can becooled and dehumidified simultaneously or in a single chamber/stage, andthis combined action reduces system size and cost as well as the numberof required components and equipment (e.g., do not require a multi-stageunit or device to cool and then to dehumidify and/or further cool withrefrigerant or the like). The combination of liquid desiccantdehumidification with indirect evaporative cooling provides very highenergy transfer rates due to evaporation and absorption. The designcreates a liquid desiccant system that does not require separateequipment for liquid desiccant cooling (e.g., a separate cooling toweror chiller). The stacked arrangements or multi-layered mass/heattransfer assemblies (or manifolded flow chambers/channels) enableultra-low flow liquid desiccant designs. This is due in part to theenhanced geometry of the assembly and its ability to decrease the liquiddesiccant's temperature to a lower temperature than achievable withtraditional cooling tower technologies. Hence, in the cooler, there arehigher concentration gradients of liquid desiccant (e.g., more than 20percentage points of lithium chloride (LiCl) and similar gradients forother desiccants), which provides the following advantages: (a) a higherthermal coefficient of performance (COP) to regenerate the desiccant(i.e., to remove water from the desiccant) for reuse in the cooler; (b)less desiccant storage requirements due to better utilization; and (c)the ability to use desiccants that are less expensive than LiCl such ascalcium chloride (CaCl), which may not be used in conventional systemsbecause their absorption properties are not as favorable as LiCl butlower temperature operation provided by the cooler embodiments describedherein makes the properties of this and other “weaker” desiccants moreacceptable or favorable.

The use of membranes as chamber sidewalls facilitates fabrication ofcounter-flow and counter-flow with pre-cooled exhaust air embodiments.Liquid desiccant containment with water molecule-permeable membraneseliminates liquid desiccant “carry over” in which small droplets ofdesiccant are passed into the air stream as is a concern with directcontact arrangements. The embodiments described herein also provideconsiderable reduction or even elimination of deposited solids duringthe process of water evaporation or adsorption (and liquid flow ratescan be maintained at levels that are high enough to further controlpotential deposits) whereas fouling leads to increased maintenance andoperating costs with prior evaporative coolers.

FIG. 1 illustrates in a schematic an evaporative cooler (or mass/heatexchanger) 100 that is useful for providing concurrent or simultaneousdehumidifying and cooling of a process or inlet air stream 120 (e.g.,outdoor or process air to be cooled and conditioned prior to being fedinto a building ventilation system). The cooler 100 is shown insimplified form with a housing shown in dashed lines, without inlet andoutlet ducts, plumbing, and/or manifolds. Also, the cooler 100 is shownwith a single mass/heat transfer stack 110 whereas in a typical cooler100 there would be numerous stacks 110 provided by repeating theconfiguration shown (e.g. by alternating the liquid passed through thechamber defined by the membrane and wall) to provide an assembly with aplurality of air and liquid flow channels or chambers to provide thedesired mass and heat transfer functions described for the stack 110.

As shown, an inlet air stream 120 is directed in a chamber or channeldefined in part by a sheet or layer of a membrane 112. Liquid desiccant124 flows in an adjacent chamber or channel on the other side of themembrane 112. The liquid desiccant 124 is contained by the membrane 112,which is permeable to water molecules in a liquid or vapor state butgenerally not to the components of the liquid desiccant 124. The chamberfor the desiccant flow 124 is also defined by a sheet or layer ofmaterial that is impermeable to fluid flow (i.e., a separation wall) 114so as to contain the liquid desiccant 124 in the chamber or flow path.The chamber for stream 120 is also defined by an opposing membrane (notshown) that is used to contain another flow of liquid desiccant. In thismanner, heat is passed or removed from the inlet air stream 120 andtransferred to the liquid desiccant flow 124 (and the desiccant behindthe opposite sidewall/membrane (not shown)). Concurrently, the inlet airstream 120 is dehumidified as water 130 is removed by passing throughthe permeable membrane 112 into liquid desiccant 124.

The liquid (or gas) desiccant 124 may take many forms to act todehumidify and cool the air stream 120 as it passes over the membrane112. Desiccant 124 is generally any hygroscopic liquid used to remove orabsorb water and water vapor from an air stream such as stream 120.Preferably, the desiccant 124 chosen would be a regenerable desiccant(e.g., a desiccant that can have the absorbed water separated and/orremoved) such as a glycol (diethylene, triethylene, tetraethylene, orthe like), a salt concentrate or ionic salt solution such as LiCl, CaCl,or the like, or other desiccants. The membrane 112 may be formed of anymaterial that functions to contain liquid desiccant 124 and, typically,coolant 126 (e.g., water or the like) while also being permeable tomolecules of water in liquid or vapor state. For example, polymermembranes may be used that have pores that are about the size or justbigger than a water molecule and, in some cases, that are also adaptedto provide water molecules with high mobility through the membrane 112.In one particular embodiment, the membrane 112 is formed from a membranematerial as described in detail U.S. Pat. No. 6,413,298 to Wnek, whichis incorporated in its entirety herein by reference. The membranematerial may also be obtained from a number of distributors ormanufacturers such as, but not limited to, Dias-Analytic Corporation,Odessa, Fla., U.S.A. The membranes 112, 118 and separation wall 114preferably also are formed from materials that are resistive to thecorrosive effects of the desiccant, and in this regard, may befabricated from a polymer or plastic with the wall 114 in some casesbeing formed of a corrosion resistant metal or alloy, which provides ahigher thermal conductivity compared with a plastic.

The embodiment 100 shown is configured for counter-flow of thepre-cooled exhaust air stream 128 (relative to the inlet air stream120). Other embodiments may use cross (at about a 90 degree flow path)or quasi-counter flow (e.g., not directly counter or opposite indirection but transverse such as a greater than 90 degree angle flowpath relative to air stream 120). The exhaust air stream 128 flows in achannel or chamber defined by a sheet or layer of membrane (e.g., secondor lower membrane) 118 and an upper membrane of another stack (notshown). The separation wall 114 and membrane 118 define a flow chamberor channel for coolant flow 126, which is typically a flow of water orthe like. Heat is transferred from the liquid desiccant 124 to thecoolant 126 through the separation wall, and the coolant 126 is cooledas heat and mass (e.g., water or other moisture 132) is transferred tothe exhaust stream 128 via membrane 118. Heat transfer is not shown butgenerally is flowing through the membrane 112 to the liquid desiccant124, through the separation wall 114 from the liquid desiccant 124 tothe coolant 126, and through the membrane 118 from the coolant 126 tothe exhaust air stream 128. The membranes 112, 118 are relatively thinwith a thickness, t_(mem), that typically is less than 0.25 inches andmore typically less than about 0.1 inches such as 100 to 130 microns orthe like. The membrane 112, 118 may have a tendency to expand outward ifunrestrained, and, in some embodiments, such as that shown in FIG. 3, adivider or “flow field” support is provided in the inlet air stream 120and exhaust air stream 128 (i.e., in the airflow chambers) to maintainthe separation of the adjacent membranes (e.g., a plastic or metallicmesh with holes or openings for air flow and a zig-zag, S or W-shaped,or other cross section (or side view) that provides many relativelysmall contact points with the membranes 112, 118). The separation wall114 also typically is relatively thin to facilitate heat transferbetween the desiccant 124 and coolant 126 such as with a thickness,t_(wall), of less than 0.125 inches or the like. The flow chambers forthe air, desiccant, and coolant are also generally relatively thin withsome applications using chambers less than 1 inch thick (or in depth)while others use chambers less than about 0.5 inches, such as about 0.25inches or less.

FIG. 2 illustrates an indirect evaporative cooler 210 utilizing themembrane/separation wall/membrane stack or assembly configuration toprovide a mass/heat transfer exchanger device in which dehumidificationand cooling occur within a single stage and, therefore, an integral orunitary device. In some embodiments (not shown), there is no desiccantside membrane or desiccant flow. Thus, these embodiments are useful forproviding an indirect evaporative cooler in which the membrane containsliquid coolant but not liquid desiccant and the membrane typically wouldnot be provided on the supply air side (or in these channels) to providebetter heat transfer surfaces with the separation wall. As shown in FIG.2, the cooler 210 includes a mass/heat transfer assembly formed fromstacks or devices 212, 230, 240 and such an assembly of stack wouldtypically be repeated to provide a plurality of inlet and exhaust air,coolant, and desiccant flow channels or chambers in the cooler 210. Asshown, each set of stacks (or layered assemblies or devices) 212, 230,240 is formed similarly to include a membrane, a separation wall, and amembrane, with the membrane being permeable to water on the molecularlevel to allow mass and heat transfer and the wall being impermeable (ornearly so) to only allow heat transfer and not mass transfer.

Specifically, the stack 212 includes an upper membrane layer 214, aseparation wall 216, and a lower membrane layer 218. Dividers or spacers(not shown) would typically be provided to space these layers apart todefine flow channels for coolant 215 and for liquid desiccant 217. Forexample, the separators may be configured to also provide a connectionto a supply line for coolant and for regenerated desiccant, provide amanifold(s) to direct flow through the various stacks 212, 230, 240, andprovide a connection to a return line for the coolant and diluteddesiccant. The stacks 230 and 240 likewise include an upper membranelayer 232, 242, a separation wall 234, 244, and a lower membrane layer238, 248. The stack 240 has coolant (such as water) 243 directed in thechamber between the upper membrane 242 and wall 244 and desiccant 246flowing between the wall 244 and lower membrane layer 248 similar tostack 212. In contrast, the stack 230 has liquid desiccant 233 directedto flow in the chamber defined by the upper membrane layer 232 and wall234 and has coolant 236 directed to flow in the chamber or channeldefined by the wall 234 and lower membrane layer 238.

The cooler 210 includes ducting and the like (not shown) to directsupply inlet air 250 through the channel or flow path between the stack212 and the stack 230. The arrangement of the stacks 212, 230, 240 andcontained fluids results in the supply inlet air 250 being passed overthe surfaces of the membranes 218, 232 that are containing liquiddesiccant 217, 233. As a result, supply outlet air 254 is output that isdehumidified as moisture in the air 250 is absorbed by the desiccant217, 233 via permeable membrane 218, 232, and the air 254 is also cooledby the interaction with desiccant 217, 233. The cooling effect in thecooler 210 is in part effected by a fraction of supply outlet air 254being redirected in the cooler 210 by ducting/manifolds (not shown) toflow as pre-cooled exhaust air 255 through the channel or flow pathbetween stacks 230, 240 to be output as warmer and moister air 258. Heatpasses from desiccant 233 through wall 234 to coolant 236 (with similarheat transfer occurring in stacks 212, 240), and the coolant 236 is ableto transfer heat and mass (e.g., water molecules) via membrane 238 tothe incoming exhaust air 255. As discussed above, the stack pattern orset provided by 212, 230, 240 would typically be repeated within thecooler 210 to create a mass/heat transfer assembly with numerous,parallel flow channels for air, coolant, and desiccant.

The cooler 210 is shown as a counter flow exchanger, but other flowpatterns may be used to practice the desiccant-based dehumidificationand cooling described herein. For example, cross flow patterns mayreadily be established as well as quasi (or not fully opposite) counterflow patterns. These patterns may be achieved by altering themanifolding and/or ducting/plumbing of the cooler as well as thedividers provided between the stacks. Additionally, the counter flowpassages may be provided integral to the stack assembly rather thanexternally as is the case in the cooler 210. For example, the cooler 310has a similar stack arrangement as shown in the cooler 210 of FIG. 2except that it includes a counterflow baffle or dividing wall 360 (FIG.3) on the end of the flow channels for inlet air 250 and exhaust air258. The counterflow divider 360 allows a majority of the cooled air toexit the stacks as supply outlet air 354 (e.g., more than about 50percent and more typically 60 to 90 percent or more of the air flow250). A smaller portion (e.g., a volume equal to the make up outdoor airor the like) is directed by divider 360 to flow between stacks 230, 240as pre-cooled exhaust air 355. FIG. 3 also illustrates the use of adivider or flow field baffle 370 that functions to maintain a separationof membranes in the stacks 212, 230, 240 (or at about their originalthickness rather than puffed out or expanded as may occur with somepermeable membranes). The dividers 370 may take many forms such as amesh with a wavy pattern (e.g., an S or W-shaped side or cross sectionalview), with the mesh selected to provide as little resistance to airflow as practical while still providing adequate strength. Also, it isdesirable to limit the number of contact points or areas with themembranes as these can block moisture transfer from the air 250 and tothe air 355.

FIG. 4 illustrates an indirect evaporative cooler 400 of one embodiment.A housing 410 is provided for supporting a mass/heat transfer assemblysuch as one formed with the stack sets shown in FIGS. 1-3 and 15-19. Asshown, the housing 410 includes a first end 412 with an inlet 414 forsupply inlet airflow 415 and an outlet 416 for exhaust airflow 417. Thecooler 410 further includes a second end 418 opposite the first end 412that provides an outlet or vent for directing supply outlet airflow 420to an end-use device or system (e.g., an inlet or supply for return airto a building). The second end 418 is also configured to redirect aportion 426 of the cooled (and, in some operating modes, dehumidified)air 426 for use in counter flow cooling of the supply inlet airflow 415.A prototype of the cooler 400 was fabricated with a stack assembly asshown in FIG. 2 with 32 desiccant channels. The prototype was testedwith 10 liters per minute (LPM) flow (or about 0.3 LPM per desiccantchannel). Coolant was provided as water at a water flow rate of about1.25 to 2.00 times the evaporation rate. The evaporation rate for thisprototype was about 1.33 gallons/ton-hr or about 5 liters/ton-hr, whichprovides a water or coolant flow rate of about 6-10 liters/ton-hr ofcooling. Of course, these are exemplary and not limiting flow rates, andit is expected that the flow rates of liquid desiccant and coolant willdepend on numerous factors and will be matched to a particular channeldesign and cooling need as well as other considerations.

An indirect evaporative cooler such as the cooler 400 using stack setsas shown in FIG. 2 (or FIGS. 15-19) may be modeled to determine theeffectiveness of the use of a permeable membrane to contain coolant andliquid desiccant. FIGS. 5A and 5B provide a diagram 500 of such modelingshowing use of stacks 212, 230, and 240 as discussed with reference toFIG. 2 to cool inlet or process air and to also dehumidify this air inthe same stage or process. The inputs to the model 500 are shown, andresults for a typical inlet air condition are provided, with results andmodeling being performed in this case with Engineering Equation Solver(EES). The numeric values shown in boxes or with squares around them areinput values (or assumed typical operating conditions), and the valuesoutside or without boxes are outputs or results of the modeling. Themodeling results shown in the diagram 500 are believed to beself-explanatory to those skilled in the heating, ventilation, and airconditioning (HVAC) arts and do not require detailed explanation tounderstand the achieved effectiveness of the embodiments using membranecontainment in indirect evaporative coolers; however, the followingprovides a graphical description of some of the results in the diagram500.

FIG. 6 illustrates a graph or diagram 610 showing the temperatures ofthe air flows in the channels between the stacks (e.g., in anevaporative cooler using such mass/heat transfer assembly describedherein). The graph 610 also shows surface temperatures along the lengthof the counterflow mass/heat exchanger (e.g., exchanger 400 with stackarrangements as shown in FIG. 2). Specifically, the graph 610 shows thetemperature of supply air with line 612, the temperature ofexhaust/purge air with line 614, the temperature of the desiccant sidemembrane surface (e.g., at the interface of the membrane and the supplyair) with line 616, the dewpoint temperature of the desiccant sidemembrane surface (e.g., at the interface of the membrane and the supplyair) with line 620, and the temperature of the water side membranesurface (e.g., at the interface of the membrane and the exhaust/purgeair) with line 618.

FIG. 7 is a graph or diagram 710 showing the humidity ratios of the airalong the length of the counterflow heat/mass exchanger. Specifically,the graph 710 shows the bulk humidity ratio of the supply air with line712, the bulk humidity ratio of exhaust/purge air with line 714, thehumidity ratio of the air in close proximity to the desiccant sidemembrane surface (e.g., at the interface of the membrane and the supplyair) with line 716, and the humidity ratio of the air in close proximityto the water side membrane surface (e.g., at the interface of themembrane and the exhaust/purge air) with line 718.

FIG. 8 illustrates a graph 810 showing with line 815 the concentrationof desiccant (in this particular modeling the desiccant is LiCl) as itflows concurrent with the supply air flow down the length of thecounterflow mass/heat exchanger. As shown with line 815, the desiccantis getting weaker as it flows through the channel between the membraneand the separation wall as it absorbs water molecules from the air,e.g., the concentration of the desiccant is dropping from about 44percent down to about 24 percent in this particular modeling example(which results from the membrane being characterized as permeable (at aparticular input rate or setting) to water molecules in the flowing airat these operating conditions).

FIG. 9 shows the process of model 500 of FIG. 5 in a psychrometric chartor diagram 910. The supply air shown with line 912 can be seen to begradually losing humidity (in kilograms water vapor/kilograms dry air orkg_(v)/kg_(da)). The supply air 912 has its temperature initially riseslightly due to the large heat flow of vapor sorption into thedesiccant. As the supply air 912 continues down the length of theexchanger (or flow channel or chamber between membrane layers or wallsof adjacent stacks containing liquid desiccant), the temperature thendrops to a cooler/drier condition than at the inlet. At the exit of theexchanger, the supply air 912 is split into two streams. The majority ofthe air is supplied to the cooled space, and the minority of the air(such as less than about 50 percent and more typically less than about30 percent of the volume) gets funneled into the exhaust/purge side (orexhaust/counterflow channels between the membrane walls containingcoolant) of the heat/mass exchanger or cooler, which is shown with theline 916. The exhaust air 916 has a low dewpoint, and, thus, it can pickup a large amount of heat evaporatively. The pre-cooled exhaust or purgeair 916 picks up water vapor (and associated heat of vaporization) fromthe wet side channel. The air 916 exits out of the unit with a muchhigher enthalpy than either the supply inlet or exit shown with line912. The diagram 910 also shows the humidity ratio and temperature ofthe supply air in close proximity to the desiccant side membrane surface(ds) with line 918.

The following table shows results in tabulated form for modeling of FIG.5 for inlet and outlet air flows. As shown, a wide range of temperaturesand humidity levels can be chosen and input into the model 500. In theconfiguration whose results are shown in the table, the equivalent wetbulb effectiveness with the desiccant flow turned off (e.g., in someoperating modes it may not be required or useful to utilize thedesiccant to dehumidify the air) would be 113 percent, which means thecooler is able to cool the supply air below the inlet wet bulbtemperature.

TABLE Inlet and outlet conditions from model runs (° C. and kg/kg) Run #T_(supply,in) T_(supply,out) T_(exhaust,out) ω_(supply,in)ω_(supply,out) ω_(exhaust,out) 1 27.7 21.11 31.55 0.0133 0.00892 0.02892 50.0 33.7 50.7 0.0319 0.0179 0.0834 3 50.0 20.7 41.0 0.0077 0.004060.0494 4 30.0 13.1 27.2 0.00262 0.00158 0.0226 5 30.0 18.9 42.55 0.02690.0137 0.0547 6 15.0 16.9 25.4 0.0105 0.00418 0.0207 7 15.0 11.9 20.00.00528 0.00203 0.0147

where LiCl Inlet Concentration=44%; flow ratio (flow exhaust/(flowexhaust+flow supply)=0.3; supply outlet face velocity=175 SCFM; andambient pressure=101.3 kPa.

The cooler 210 of FIG. 2 (or FIGS. 15-19) may be thought of as adesiccant-enhanced, indirect evaporative cooler that utilizes amembranes or layers of membrane material that is permeable to watermolecules to provide desired liquid containment. A standardpsychrometric chart (such as one at 14.7 psi ambient pressure and othertypical parameters) may be used to view lines of equal sensible heatratios (SHRs) originating at a typical room setpoint. For vaporcompression dehumidification, a SHR of less than about 0.7 is difficultto attain without reheat (e.g., given reasonable evaporatortemperatures). Also, it is psychrometrically impossible to attain a SHRof less than about 0.6 without reheat, and attempting such a SHR oftenleads to frozen evaporator coils that require defrost cycles. Thedesiccant-enhanced, indirect evaporative cooler, such as shown in FIG. 2at 210, addresses this problem with a unique, new process (as has beendescribed above and is presented in more detail below).

It may be useful at this point to review the process with reference toFIGS. 2 and 3. FIGS. 2 and 3 show diagrams describing the inner flowchannels of the unit or assembly for use in an evaporative cooler 210,310. The mixed return/outdoor air is shown by the arrow 250 (e.g.,return air from a conditioned space along with outdoor make up air suchas 400 cfm/ton supply and 175 cfm/ton outdoor air or the like). The air250 is dehumidified by the desiccant 217, 233 through the membrane 218,232. This lowers both the dew point and temperature of this air streamuntil it is output at 254 or 354. At the exit of the supply air passage(between the liquid desiccant-containing membranes), a portion of theair is fractioned off as shown with arrows 255 and 355 and sent throughan adjacent passage (between the coolant-containing membranes 238, 242)which picks up moisture from the water layer 236, 243 through themembrane 238, 242. The heat of evaporation is a source of cooling thatacts to remove the sensible heat and heat of absorption from the supplyair stream 250. This air is then exhausted (purged) out at 254, 354.

The heat exchanger configuration shown at 400 in FIG. 4 has been builtin the laboratory and was modeled as shown in FIG. 5. Other options forflow/housing designs are shown in configuration with the cooler 1000 ofFIG. 10 and the cooler 1100 of FIG. 11. The cooler 1000 is shown to havea housing 1010 with a first portion or end 1012 and a second portion orend 1020. The first portion 1012 is configured with inlets or vents forreceiving supply inlet airflow 1013 as well as input exhaust airflow1014, and the first portion 1012 also includes vents or outlets foroutputting exhaust airflow 1015 from the unit 1000. The second portion1020 is configured (e.g., with manifolds and other components to directair flow) with outlets for supply outlet airflow 1022 with a portion1025 being redirected back into the housing 1010 as shown at arrows 1027to provide counterflow for a fraction of the channel provided for supplyinlet airflow 1013 (with exhaust airflow 1014 provided as a cross flowin the other or initial portion of the channel) and then this air isexhausted from the housing portion 1020 at 1028. The input exhaustairflow 1014 may be return air to be exhausted or outdoor air (e.g.,from the building space). This approach 1000 improves the efficiency byutilizing a smaller purge airflow 1025, 1027, and it is typicallypreferred to limiting purge air flow to increase or maintain desirableefficiency.

Referring again to FIG. 4, operation of the cooler 400 is expected tohave the cooling process shown in the psychrometric chart 910 of FIG. 9.As shown, line 912 represents the supply air flow while line 916represents the purge air flow stream. The desiccant side air boundarylayer is represented with line 918. The chart shows graphically how thedehumidification driver for the cooler 400 is advantageously utilized toprovide a more effective cooler. The cooler 400 may use even a weakdesiccant such as CaCl solution to provide significant dehumidification,and this is due in part to the cold temperatures that are achieved withthe configuration of the cooler 400 that allow weak desiccants to attainhigh dehumidification potential.

The configuration shown with cooler 1000 of FIG. 10 was modeled todetermine the desirability of its performance, and the results areprovided in psychrometric chart 1200 of FIG. 12. In the chart 1200, line1210 represents supply air, line 1212 represents ambient exhaust air,line 1214 represents desiccant side surface temperatures, line 1220represents the supply air post cooling, line 1224 represents the purgeair post cooling, and line 1230 is the sensible heat ratio (SHR) line inwhich the load on the building follows. So, for example, a building willhave 0.67 units of sensible heat and 0.33 units of latent heat added tothe space to arrive at the return air condition, which is the middlediamond at 80° F. and about 70 grains/lb, and that point may beconsidered the return air condition. The first point of line 1210 is the“mixed air” condition, which is a 30/70 mixture of outdoor air andreturn air. The two-stage approach to cooling provided by cooler 1000allows the process to be split into two distinct sections ofdehumidification plus a post cooling stage (e.g., sensible cooling onlystage in which, for example, there is no desiccant layer anddehumidification and only evaporative cooling is provided). The cooler1000 is, of course, only one example of numerous configurations that maybe implemented to provide two or more stage cooling using the membranecontainment features described herein, and it shows the possibility ofattaining nearly any SHR desired (e.g., in this case, a SHR of about0.67). In the modeling to provide the chart 1200 (FIG. 12), a 1 cubicfoot core (or mass/heat transfer assembly) was used with 176 SCFM, and aflow ratio of about 0.3 (e.g., 30 percent purge and 70 percent supplyair). Also, the return air was at 80° F. and 40 percent relativehumidity, ambient air was at 86° F. and 60 percent relative humidity,and the liquid desiccant fed into the assembly was 44 percent LiCl (butother desiccants such as solutions of salt (such as, but not limited to,halide salts) and water that are about 20 to 40 percent salt by weightmay be used). The assembly was able to provide 0.5 tons of buildingcooling with just this 1 cubic foot at about 7 Btu/lb. As can beappreciated from this example and modeling, the use of membranes tocontain desiccant and coolant (e.g., to contain liquids) enable indirectevaporative coolers to be produced that are much more compact than priordesigns, that are easier to maintain (e.g., have less or no foulingissues), and that are more efficient in producing cooling (e.g., withsimultaneous dehumidification and cooling to provide an evaporativecooler that can condition as well as cool process air).

FIG. 11 illustrates an evaporative cooler 1100 providing anothercounterflow arrangement in which the counterflow cooling air (orpre-cooled supply air) is directly opposite in direction but only for aselected length (such as half to 80 or 90 percent or more of the length)of the stacks or flow chambers (e.g., when full counterflow is notrequired or desired). As shown, the cooler 1100 includes a housing 1110containing a plurality of stacks or sets of stacks configured as amass/heat transfer assembly (as discussed above) with alternating flowchannels for supply inlet airflow 1112 and for counterflow air (e.g.,redirected supply outlet airflow 1114). The housing 1110 includesventing and/or manifolding for directing the supply inlet airflow 1112(e.g., outdoor make up air and return air) into channels betweendesiccant containing membranes and to output the cooled and, often,dehumidified supply outlet airflow 1114. The cooler 1100 furtherincludes ducting, manifolding, and the like for redirecting a fractionof the supply outlet airflow back into the housing 1110 to providecooling counterflow air as shown at 1116 (e.g., into flow channelsbetween coolant containing membranes). The counterflow air 1116typically does not travel along the entire length of the housing 1110but is, instead, discharged out a side vent at some point along achannel length (e.g., at a distance about 60 to 80 percent of thelength). Such a configuration is useful to tune a cooler 1100 forparticular operating environments (e.g., to provide a desired amount ofcooling to the supply outlet airflow based on outside air temperaturesand humidities and other operating parameters).

The stack and membrane technology described herein are readilyapplicable to a number of indirect evaporative cooler designs (with andwithout use of liquid desiccant for dehumidification) and applications.However, it may be useful to discuss the use of the technology within anair conditioning or HVAC system with the belief that those skilled inthe art will readily understand that the technology is useful in manyother such systems. FIG. 13 illustrates a simplified air conditioningsystem 1300 in which the membrane technology may be provided to disclosedesiccant dehumidification and evaporative cooling to condition airwithin a building 1310 (e.g., a residential or commercial building orother structure requiring conditioned and cooled air). As shown, thesystem 1300 includes a cooler 1320 with a housing 1322 that is used tohouse a membrane stack assembly, such as described above with referenceto FIGS. 1-12 and below with reference to FIGS. 15-20. A fan or blower1324 is provided to draw in outside or make up air 1325 and move returnair 1326 from the building 1310. The fan 1324 pushes these two airstreams as inlet supply air through the stacks as described above (e.g.,adjacent liquid desiccant contained in membrane in embodiments providingdehumidification or adjacent separation walls in embodiments with justevaporative cooling). The cooled (and, typically, conditioned air isoutput at 1330 as supply to the building 1310 and a portion is returned1332 as purge or pre-cooled exhaust air that passes on the coolant orevaporative cooling side of the stacks in housing 1322 and then out asexhaust 1328. Coolant is provided in the form of a water supply anddrain 1334 to the housing (and through the stack assembly), and liquiddesiccant is provided at 1338 as supply and drain. The desiccant 1338 isregenerated with a regenerator system 1340 including, in this example, adesiccant boiler 1342.

The desiccant enhanced indirect evaporative cooler (DE-IDEC) 1320 is theportion of the system 1300 that takes strong desiccant and water toprovide cooling to building 1310. The system 1300 provides both sensibleand latent cooling to building 1310 on demand and in proportion to thedemand, e.g., the system 1300 can provide cooling in the form of 100percent sensible, 100 percent latent, or any combination thereof. TheDE-IDEC 1320 uses some portion of outdoor air 1325 with equal exhaustair 1328 to reject the heat load outside of the building 1310. TheDE-IDEC 1320 itself can sit inside or outside of the building envelopebecause it has no wet surfaces and the liquid streams 1334, 1338 areclosed loop. This makes system 1300 acceptable for indoor use and forplacement of cooler 1320 inside the building 1310. The water source (orcoolant source, not shown) for water or coolant 1334 is not required tobe potable, and the system 1300 is compact enough to be acceptable bybuilding managers. The electricity usage is much less than that oftypical vapor compression systems or units (e.g., less than 0.2 kW/tonpeak compared with 1.2 kW/ton typical for conventional compressionunits).

The regenerator 1340 is another of the significant components to theoperation of the system 1300. This unit 1340 takes the weakeneddesiccant from the DE-IDEC 1320 and applies heat with boiler 1342 (seelist of heat sources below) to drive off the moisture contained in thedesiccant 1338. The result is a desiccant 1338 that has higher saltconcentration and can be re-used by the DE-IDEC 1320 (e.g., in themembrane contained/defined flow channels adjacent to supply inlet air1325, 1326). A list of heat sources suitable for desiccant regenerationmay include: (a) gas or other fossil fuel; (b) solar heat; (c) wasteheat from any waste heat stream such as combine heat and power plant;and (d) waste heat from a condenser unit originating from a vaporcompression cycle.

The inventors performed a test of a prototype fabricated similar to thecooler shown in FIG. 4 with a stack assembly such as shown in FIG. 2.FIG. 14 provides results of the testing for this proof of conceptprototype that was constructed and tested at 104° F. and 93 grains/lbinlet air. The prototype was tested with and without desiccant flow, butwith membranes provided to define liquid desiccant flow channels.Without the desiccant flow, the indirect evaporative cooler had awet-bulb effectiveness of 73%. When desiccant was turned on (with 41%LiCl solution as the desiccant), the effectiveness was 63% and had 12grains/lb of dehumidification. This resulted in a sensible heat ratio of0.73. The prototype did not reach model expectations as explained above,and this was likely due to prototype defects creating non-uniform air,water, and desiccant flow distribution.

It was recognized that use of the membrane to contain the liquiddesiccant and separate it from air flow is desirable in most if not allmass transfer/heat exchanger assemblies. For example, with reference tothe indirect evaporative cooler 100 of FIG. 1, the membrane 112 is usedto block flow of the liquid desiccant 124 into the inlet air stream 120while concurrently allowing water molecules 130 to flow from the inletair stream 120 to the desiccant 124 to dehumidify and cool the inlet orprocess air 120.

However, it was further determined that the second membrane 118 is notneeded to practice many aspects of the evaporative coolers describedherein. Particularly, an indirect evaporative cooler may be provided inwhich each stack only includes a single water-permeable membrane (suchas membrane 112) while coolant flow is provided on the opposite side ofa separation wall (such as wall 114) through other techniques such as byproviding a flocking sheet or layer (or wicking element) on theseparation wall 114 opposite the side of the wall defining the liquiddesiccant flow chamber/channel. The stack may be arranged vertically insuch embodiments of the evaporative cooler to make use of gravity toencourage coolant flow from the top to the bottom of the stack in thewicking layer. In other cases, though, the wicking layer or flocking maybe provided on a top or bottom side of a separation wall (a horizontalstack arrangement) with capillary action (or other mechanisms) used toobtain a desired coolant flow through the stack.

FIG. 15 schematically illustrates an indirect evaporative cooler (ormass/heat exchanger) 1500, which may be used in place of the evaporativecooler 100 shown in FIG. 1. The cooler 1500 may be thought of as amodification of the cooler 100 with retained components or elementshaving like reference numerals in FIGS. 1 and 15. Particularly, theevaporative cooler 1500 is useful for providing concurrent dehumidifyingand cooling of a process or inlet air stream 120. This is achieved withone or more mass/heat transfer stacks 1510. As shown, the inlet airstream 120 is directed to flow in a chamber or channel defined in partby a sheet or layer of a membrane 112, which may take the form describedabove for stack 110. Liquid desiccant 124 flows in an adjacent chamberor channel on the other side of the membrane 112. The chamber for thedesiccant 124 flow is also defined by a separation wall 114, which, asdescribed above, is impermeable to fluid flow so as to contain theliquid desiccant 124. The chamber for air stream 120 is also defined byan opposing membrane (not shown) that is used to contain another flow ofliquid desiccant (e.g., a membrane of another stack configured similarto stack 1510).

As with cooler 100 of FIG. 1, the evaporative cooler 1500 is configuredfor counter-flow of the pre-cooled exhaust air stream 128 (relative tothe inlet air stream 120). In contrast to the cooler 100, though, theexhaust air stream 128 flows in a channel or chamber defined on one sideby a wicking layer or flocking element 1520 and on another side by anupper element of another stack (not shown, but may be another wickinglayer or a membrane).

Significantly, the wicking layer or flocking 1520 is attached to a sideof the separation wall 114 and acts to wick or guide flow of a volume ofcoolant 126 in the stack 1510. In other words, the second membrane 118of cooler 100 is removed as it is not needed to define a coolant flowchannel/chamber. Instead, the wicking layer 1520 may be thought of asdefining a channel or flow path for the coolant 126, which is shown tobe counter to the exhaust air stream 128. The air in stream 128 is incontact with the wicking layer 1520 and the coolant 126.

The coolant 126 may be a flow of water or the like, and heat istransferred from the liquid desiccant 124 to the coolant 126 through theseparation wall 114. The coolant 126 flowing or being wicked by wickinglayer 1520 is cooled as heat and mass (e.g., water or other moisture132) is transferred to the exhaust air stream 128 directly rather thanthrough a membrane as in cooler 100 of FIG. 1. Heat transfer is notshown in FIG. 15 but generally heat is flowing through the membrane 112to the liquid desiccant 124 via water 130 and then through theseparation wall 114 from the liquid desiccant 124 to the coolant 126,and then from the coolant 126 to the exhaust air stream 128.

Capillary action may support flow of coolant 126 in wicking layer 1520when the stack 1510 is arranged in a horizontal configuration, but someembodiments will position the stack 1510 including the separation wall114 and attached/contacting wicking layer 1520 to be vertical such thatgravity facilitated coolant flow 126 from the top to the bottom of theevaporative cooler 1500. As with the stack 110, the stack 1510 may beprovided in multi-stack assemblies/coolers such as the cooler 210 withthe stack 1510 being used to provide, or in place of, stack 230 (and/orother stacks 212, 240). In such an arrangement, the flow channel for theexhaust air stream 128 typically would be defined by facing but spacedapart wicking layers 1520 on separation walls 114 (e.g., spaced apart,flocked surfaces of two separation walls).

A variety of flocking materials may be used to implement the wickinglayer 1520 on separation wall 114. The wicking layer 1520 acts to spreadout or disperse the flowing coolant 126, e.g., to avoid rivulets offlowing coolant, which enhances heat transfer from the wall 114 and alsomass/heat transfer to exhaust air stream 128 in the adjacent flowchamber/channel of stack 1510. The flocking material of the wickinglayer 1520 also acts to impede gravity to get a slower flow in verticalconfigurations. The thickness of the layer 1520 may vary but in somecases may be approximately 0.015 inches thick while other usefulimplementations may use flocking in the range of 0.005 to 0.05 inches inthickness. Exemplary flocking for the wicking layer 1520 include: (a)knitted nylon fabric; (b) polypropylene woven or non-woven fabric; and(c) adhesive-backed flocking fibers (typically polyester orpolypropylene), e.g., the layer 1520 may include fibers standing upalong (or arranged transverse to) planar surface of wall 114 and mayhave lengths of 0.01 to 0.05 inches or more. In some embodiments, thewicking layer 1520 may be provided by one or more fabrics coated with ahydrophilic coating. While in other cases, the layer of wicking material1520 is created with a hydrophilic coating on a surface of theseparation wall 114.

While a wide variety of materials may be used in layer 1520, there are anumber of wicking or flocking characteristics that may be desirable foroperation of the cooler 1500. The wicking surface of layer 1520 providesa method or mechanism to evenly spread either desiccant or water (asshown in FIG. 15) over a surface (e.g., surface or side of wall 114).The wicking surface impedes the forces of gravity on the flowing liquidto slow the flow rate down to a range of about 5 to 50 inches perminute, with some useful implementations using a flow of about 20in/min. The flocking also enables low total flow rate of water to beapplied. The total flow rate of water or other coolant enables flowrates that are between 1.2 to 4.0 times the evaporation rate of water(or other coolant). Typically, this flow would be set based on waterquality that is being used and would be 1.2 to 2 times the evaporationrate. In another embodiment, the flow rate of water may be set higherthan in the above examples by use of re-circulating the water. In thiscase, the water flow rate may typically be 4 times the evaporation rateand could be in the range of 3-10 times the coolant evaporation rate.

As shown in FIG. 15, indirect evaporative cooler 1500 provides a channelpair where a first airflow 120 is cooled and dehumidified by waterabsorption 130 through the vapor permeable membrane 112 to the liquiddesiccant 124. The second airflow (in the second channel of the channelpair provided by stack 1510) 128 removes heat from the first airflow 120by the evaporation of water 132. The water/coolant 132 is containedwithin a flocked or wicked surface (which provides layer of flocking1520) on wall 114 opposite the flow channel for liquid desiccant 124.The evaporation of water/coolant 132 from the flocked or wicked surfaceof wall 114 removes heat from the first airflow 120 by heat conductionand convection through the membrane-desiccant-separation wall assemblyor stack 1510.

Generally, the cooling process or method provided by operation of anevaporative cooler (such as cooler 1500) involves receiving an input orprocess air stream. This process air stream undergoes dehumidificationin a first section or portion of the evaporative cooler (i.e., thedesiccant-contained dehumidification section), and this is followed bysensible cooling in a second section (i.e., indirect evaporative coolingsection). As shown herein, though, dehumidification and sensible coolingmay occur in a single or integral section or portion of the cooler tooccur concurrently. The process air is then delivered to a work space orindoor area for use in cooling a space while the purge/exhaust air isused to remove heat from the coolant and is output/discharged from thecooler.

In some cases, it may be desirable for an indirect evaporative cooler tobe provided with a humidification section. This would allow the abovecooling method/process to be modified to include a step after sensiblecooling in which the process air is humidified adiabatically to furtherdrop the temperature of the air prior to output from the indirectevaporative cooler into a work space or building space. In someembodiments, humidification is provided by having the sensibly cooledair flowing in channels/chambers with one or both sidewalls defined byvapor permeable membranes. Particularly, the indirect section (indirectevaporative cooler) may be followed by a section that provides directevaporative cooling, which also humidifies. This acts to further reducethe temperature of the outlet stream to provide higher sensible cooling,but such higher cooling comes at the expense of providing less latentcooling (dehumidification). Such additional cooling is shown with line2025 in the psychrometric chart 2200 of FIG. 22, where the air is movedfrom an air state “2” to an air state “2.5” (with this chart 2000explained in more detail in the following description). The particularmethods or mechanisms used to provide direct evaporative cooling may beperformed in many ways to practice such a cooler.

In other cases, though, a flocked surface may be used in thehumidification section. For example, FIG. 16 illustrates ahumidification section (or portion of such a stack/assembly) 1600 inwhich a separator or separation wall 1610 is provided to definesidewalls of two adjacent flow channels for process air 1614 (i.e., airthat has been sensibly cooled in an upstream section of an evaporativecooler). Both sides of the wall 1610 have been covered with flocking orwicking material to provide a top wicking layer/element 1620 and abottom wicking layer/element 1622 that are wetted (such as with water)to provide a moisture source or coolant 1630, 1632 for humidification asthe air 1614 flows over the wetted surfaces of layer 1620 and to providea heat/mass transfer to exhaust air stream 1640 (but the bottom flockingsurface/layer 1622 may be omitted in some embodiments). Note, the airstreams 1614 and 1640 (and 1710, 1740 below) may both be supply air.

FIG. 17 illustrates another humidification section 1700 that may be usedin an indirect evaporative cooler (downstream from the sensible coolingsection). In the humidification section 1700, a sensibly cooled airstream 1710 flows over a vapor permeable membrane 1714 separating theair flow 1710 (or the channel it flows within) from a flow of water orthe like 1716 (or the channel in which it flows). The water flow channelor humidification source is defined on the other side by a firstside/surface of a separation wall 1720. The other side of the separationwall 1720 along with a vapor permeable membrane 1730 defines a channelor chamber for flow of a coolant 1724. The humidification section 1700further includes another or second channel or chamber in which exhaustair 1740 flows along the other side of the vapor permeable membrane 1730and to remove heat from the evaporative cooler containing humidificationsection 1700.

At this point, it may be useful to describe a two-stage indirectevaporative cooler 1800 with reference to FIGS. 18-20. These figuresshow graphically how the cooler 1800 works on three levels: (1) FIG. 18illustrates a heat exchange schematic showing general air, water, anddesiccant flows; (2) FIG. 19 illustrates a channel pair graphic orschematic that shows an air channel pair and location of membranes andwicked water surfaces in the first stage and in the second stage heatand mass exchangers; and (3) FIG. 20 provides a psychrometric chart 2000showing each air state in the cooler 1800.

FIG. 18 schematically illustrates a two-stage indirect evaporativecooler 1800 and its air flow pattern during operation. Air states arenumbered in FIG. 18, and these air state numbers are repeated in FIGS.19 and 20 (as are reference numerals to components shown in both FIGS.18 and 19). In this discussion, air streams may be referred to ordescribed as moving from one state to the next such as air stream “1” to“1.5” is the stream of air moving from a first air state to a second airstate as the air is dehumidified.

The cooler 1800 is configured in two distinct stages or assemblies 1810and 1850 providing a first-stage dehumidifier and a second-stageindirect evaporative cooler. As shown, the dehumidifier 1810 is made upof a number of stacks 1814 (as discussed above and shown in FIG. 19).Each stack 1814 defines a flow channel or chamber for inlet or processair 1820 to flow through the dehumidifier 1810 and be output to thesecond stage 1850 as dehumidified air 1822. The stacks 1814 also defineflow paths for and act to contain liquid desiccant 1816 in thedehumidifier 1810 (e.g., LiCl, CaCl or the like at 35 to 40 percent byweight at a flow rate of about 0.34 gallons/minute per space coolington). Further each stack 1814 defines, with a pair of spaced apartwicking layers or surfaces on separation walls wicking or flowingwater/coolant 1818, flow channels or pathways for exhaust air 1826(input at air state “3”) to flow through the dehumidifier 1810 andremove heat from the liquid desiccant 1816 and be output at 1828 (at airstate “4”).

The first-stage dehumidifier 1810 is a cross-flow heat and massexchanger between two air streams 1820/1822 and 1826/1828. Desiccant1816 and water 1818 flow vertically and are gravity driven. The liquiddesiccant 1816 is contained by a polypropylene microporous membrane orother vapor permeable membrane (e.g., a Z-series from Celgard LLC oranother distributor/manufacturer). In some implementations of cooler1800, nozzles may be used to spray a high water flow rate (water 1818)that creates a two-phase flow of water and outdoor air in air stream1826/1828 (air states “3” to “4”). The dehumidifier 1810 may be designedto provide a low water flow rate that is spread by wicked surfaces incontact with the air stream 1826/1828. In some embodiments, a watersidemembrane may be used for controlling biological growth because itcreates a barrier that blocks organisms from implanting or growing ontowet surfaces.

The second-stage or indirect evaporative cooler 1850 is formed with anassembly or number of stacks 1854 (as shown in FIG. 19). Each stack 1854defines a flow path or channel for dehumidified air 1822 to flow throughthe evaporative cooler 1850 to be output as cooled/dehumidified supplyair 1860. Further, the stacks 1854 and/or manifolds or other portions ofevaporative cooler 1850 define flow paths/channels for a portion of thesupply air 1862 to be returned to flow through the cooler 1850 and beexhausted at 1866 after removing heat from the air stream 1822/1860.Further, the stacks 1854 provide flow paths or channels for coolant(e.g., water) 1858 such as via gravity flow in wicking layers onseparation walls. The second stage 1850 is designed as a counterflowindirect evaporative cooler. In testing of some embodiments, the stage1850 has a wet bulb effectiveness measured at 120 to 128 percent at thedesign mass flow rate. For both stages 1810, 1850 the water 1818, 1858was gravity driven and provided at a low flow rate distributed acrossthe heat transfer surfaces of stacks 1814, 1854 by a wicking material orthickness of flocking.

Top views of exemplary implementations of the stacks 1814 and 1854 ofthe stages 1810, 1850 are shown in FIG. 19 (with repeated components andflows labeled with like reference numbers). As shown, the first stagestack 1814 provides a pair of air flow channels: a first channel/chamberfor ventilation or input air 1820 (that typically includes a volume ofreturn air 1821 from the cooled space) and a second channel/chamber forexhaust air 1826 flowing into the page (cross flow in this example). Thefirst channel is defined by a first wall assembly formed of a separationwall 1960 (e.g., a plastic or metal sheet) and a vapor permeablemembrane 1962, which faces the air stream 1820, 1821. A flow of liquiddesiccant 1816 is contained within the wall assembly provided byseparator 1960 and membrane 1962. The first channel is further definedby a second wall assembly formed of a separation wall 1966 and anothervapor permeable membrane 1964. Again, the membrane 1964 faces or isexposed to the air stream 1820, 1821 and a flow of liquid desiccant 1816is provided and contained between a side/surface of separation wall 1966and the membrane 1964.

The second or paired air flow channel of first stage stack 1814 isdefined by the other/opposite side of the separation wall 1966 uponwhich a wicking layer 1970 is provided. The wicking layer 1970 wickscoolant/water that is directly in contact with flowing exhaust air toallow heat to be released from liquid desiccant 1816 and air stream1820, 1821. The second air flow channel is further defined by anotherseparation wall 1974 (which may be a top wall of a next stack), andanother wicking layer 1972 of flocking or wicking material is providedon the surface/side of the separation wall 1974 facing the wicking layer1970. Coolant such as water is wicked or gravity fed through the wickinglayer 1972 as the exhaust air flows through the stack 1814.

With regard to the second stage stack 1854 of the indirect evaporativecooler 1850, a flow channel is provided for air stream 1822. Thischannel is provided by a side/surface of a separation wall 1980 and aspaced apart second separation wall 1982. A second flow channel isprovided in stack 1854 into which a portion 1862 of the supply air 1860is returned into the stack 1854 to remove heat and be exhausted at 1866.A second air flow channel/chamber is defined by the opposite side ofseparation wall 1982, which is covered with flocking/wicking material toprovide a wicking layer 1984. Water or coolant is gravity fed throughthis layer 1984 during use of the stack 1854 in a cooler assembly. Thesecond flow channel for air stream 1862 is further defined by a secondwicking layer 1988 provided on a facing side or surface of an additionalseparation wall 1990. As discussed throughout, numerous first and secondstage stacks 1814, 1854 would be assembled or stacked upon each other toform a two-stage cooler 1800.

FIG. 20 is a psychrometric chart 2000 illustrating the thermodynamics ofthe cooling processes provided by operation of the cooler 1800. Thereturn air state is shown at 2060 while the state of the liquiddesiccant is provided with line 2050 in the chart 2000. Line 2010 showsthe thermodynamics as the incoming or supply air moves from air state“1” to air state “1.5” (as shown in FIGS. 18 and 19) and is dehumidifiedusing the liquid desiccant contained in the vapor permeable membranes inthe first stage dehumidifier 1810. Line 2020 illustrates thermodynamicsof the dehumidified air as it passes through the second stage indirectevaporative cooler 1850 and moves from air state “1.5” to air state “2”and is subject to sensible cooling. Line 2030 shows the thermodynamicproperties of the return air 1862 that is passed back through thesecond-stage cooler 1850 and is then output as purge or exhaust air1866. Line 2040 illustrates thermodynamic properties of exhaust airstream 1826 to 1828 (e.g., outside air) as it passes through thefirst-stage dehumidifier 1810. As shown in the chart 2000, the air to besupplied to a building space was dehumidified and was also reduced froman original temperature between 80 and 85° F. to about 60° F., which isuseful for cooling many residential and commercial spaces.

The cooler 1800 may be modified by adding a direct evaporative sectionor stage as shown in FIG. 21. In the cooler of FIG. 21, the cooledsupply air 1860 from the second stage 1854 is output to the directevaporative stage 1999 where the air 1860 undergoes humidification andfurther cooling before being discharged at 1997 in air state “2.5.” Asshown with the psychrometric chart 2200 of FIG. 22, direct evaporativecooling may be provided as shown with line 2035 to further reduce thetemperature of the outlet air stream (but, as shown, the air stream isalso humidified) as the air moves from air state “2” to air state “2.5.”The direct evaporative stage may be integrated into the second stagedevice 1854 or provided as a separate device (e.g., with reference toFIG. 18, a separate heat and mass exchanger in the cooler 1800downstream of evaporative cooler 1850 or be integrated into evaporativecooler 1850). As shown in FIGS. 20 and 22, the air stream is cooled atleast 15° F. such as to a temperature below 60° F. (e.g., an outlet airstream is produced with a temperature between 50 and 60° F. or thelike). Also, as shown, dehumidifying is achieved with a relatively smallincrease in air stream temperature, e.g., an increase of less than about5° F.

As shown, the supply air 1860 flows in channels defined by separationwalls 1991, 1993, and 1995 with wicking material or flocked surfaces1992, 1994 facing into each channel. In this way, water may be caused toflow next to the air 1860 to provide humidification to the output supplyair 1997 (cooled and humidified to air state “2.5” as shown in FIG. 22).Air 1997 is colder than air 1860 from the second stage 1854, thereforeless energy is required to provide a desired level of cooling.

The cooler 1800 (FIG. 18) may be assembled and implemented in a varietyof ways to practice the cooling methods and techniques described herein,but it may be useful to describe one tested assembly or cooler. In thefirst-stage, flutes were created by extrusion to form the coolantairstream 1826 to 1828 (state “3” to state “4”). Water 1818 wasdistributed via flow nozzles at the top of the dehumidifier 1810 (e.g.,in the airstream 1826 plenum) and mixed with airstream 1826 to 1828,which ran vertically downward. Some water evaporated as it traveledthrough the dehumidifier 1810, but most was collected at the bottom ofthe airstream 1828 plenum. Louvers in this plenum were used to separatethe water droplets from the airstream. Because this design did not havea mechanism to hold up the water internal to the flutes (e.g., wickedsurfaces), this configuration used a water flow rate that wassignificantly higher than the water evaporation rate. Thus, a waterreservoir and pump were used to return the water from the collectionsump to the top flow nozzles.

The unbacked vapor permeable membrane was welded to theflutes/extrusions. A liquid manifold distributed desiccant to the spacebetween the membrane and the flutes/extrusions. Air gaps on airstream1820 to 1822 (air state “1” to air state “1.5”) were maintained bystrips of spacers with the extruded flutes oriented parallel to theairflow. The design also incorporated spacers that mixed the airstreamto enhance heat and mass transfer. Flutes were used to form the channelsfor airstream 1822 to 1860 (air state “1.5” to air state “2”). A nylonwick was applied to the outer walls of the separation wall/plasticsheets. These subassemblies were then stacked with spacers between eachto form the channels for air flow 1862 to 1866 (air state “2” to airstate “5”). A low flow of water 1858 was distributed into thesecond-stage channels from the top. The nylon wick had sufficient waterupkeep to allow this flow rate to be marginally above the waterevaporation rate. Thus, a solenoid valve controlling domestic cold watermay be used to distribute water. Purge water was collected at the bottomof the plenum of air stream 1866, at which point it was directed to adrain.

Wicked surfaces provide a number of advantages for the indirectevaporative coolers described herein. The wicking ensures that the wallsare fully wetted and that there is no lost evaporation area. The waterfeed rate can be held to a factor of 1.25 to 2 times that of theevaporation rate. This technique allows for “once-through” water use.The water that drains off the heat and mass exchanger is concentratedwith minerals and can then be drained away. A sump and pumping systemare not required, which improves energy performance and eliminatessump-borne biological growth. A simple controller can periodically usefresh (low concentration) water to rinse the heat and mass exchanger(such as cooler 1800) and clear any built-up minerals. Air streams 1822to 1860 and 1862 to 1866 are in counterflow in the second-stage 1850. Asensitivity analysis showed that the cooling effectiveness could bereduced by as much as 20 percent if proper counterflow was not achieved.Air stream 1822 to 1860 flowed straight, through extruded flutes, butairstream 1862 to 1866 used a 90-degree turn before exiting the secondstage 1850. Computational fluid dynamics software may be used design anair restrictor to ensure proper counterflow of air stream 1862 to 1866.

Likewise, the stacks including the membranes and wicking material may beformed in a variety of ways to implement a mass and heat exchanger ofthe present description (such as cooler 1800). The construction of oneprototype revolved around laminated layers of polyethylene terephthalate(PET) plastic that were adhered with layers of acrylicpressure-sensitive adhesive. Although this assembly method may noteasily be scaled to high-volume manufacturing, the achievable geometriesare nearly ideal and, therefore, appropriate for prototypes. Thisenabled the creation of a prototype with parallel plate geometry thatincluded airside turbulators to enhance heat and mass transfer onairstreams. Another prototype was built using layers of extrudedpolypropylene (PP). It is likely that formed aluminum sheets may be usedto create a parallel plate structure to implement a cooler describedherein. For example, the aluminum sheets may be corrugated to form awavy flow channel, which would increase heat transfer by the waviness ofthe channel (which promotes mixing of the air stream and impingement ofthe air into the separator plate wall) and also act to reinforce thestructure by giving the sheets/plates increased rigidity. Such anarrangement may work better in the second stage where there is nodesiccant (since the desiccant may corrode the aluminum).

For the first-stage 1810, the laminated layers enabled the use of wickedsurfaces in the air stream 1826 to 1828 channels. For the spacer, anoff-the-shelf expanded aluminum grating was used, and the spacer wasused in channels for air stream 1820 to 1822 and air stream 1826 to1828. The design of the stacks such as stack 1814 used expandedpolypropylene hydrophobic membrane backed with a nonwoven polypropylenefabric to add strength. The backing reduces vapor diffusion through themembrane but increases tear resistance. The backing was oriented to theairside gap, where tears can originate from abrasion by foreign objectsor the aluminum spacer. A desiccant manifold was developed that usedlaminated layers of plastic and adhesive to effectively and evenlydistribute liquid desiccant behind the membrane. The second stage 1850used laminated construction but, with minimal spacers to create laminarflow, used parallel plate air channels. The design used strips asairflow spacers and wicked surfaces on the wet side of the heat and massexchanger 1800.

The above description concentrated or stressed designs of heat/masstransfer assemblies for use in providing unique indirect evaporativecoolers. Those skilled in the art will recognize that the coolersdescribed can readily be included in more complete HVAC systems forresidential and commercial use. Such HVAC systems would include plumbingand components to circulate liquid desiccant to and from the cooler atdesirable and controllable flow rates. These systems would also includea regenerator for the desiccant (e.g., one that heats the liquiddesiccant to remove absorbed water such as heat provided by solarpanels, electrical heaters, or the like). The regenerator also mayinclude a sump and lines for recovering potable water from thedesiccant, and storage would be provided for the desiccant prior to itbeing pumped or fed to the cooler. Portions of the system that come intocontact with the desiccant typically would be fabricated of corrosionresistant materials such as certain metals or, more typically, plastics.The HVAC system would also include ducting and other components such asfans or blowers for moving the return air from the building through thecooler and back to the cooled spaces, for moving make up air through thecooler and into the cooled spaces, and for discharging any purge orexhaust air. A coolant supply system with piping and pumps/valving (asnecessary) would also provide coolant such as potable water to thecooler stacks (e.g., channels between membranes and separation walls).

The embodiments shown typically discussed ongoing use of the liquiddesiccant to dehumidify the supply or process air. However, in manyoperating conditions, the cooler may be operated without desiccant flow,and these operating conditions may be considered “free evaporativecooling” conditions (or zones on a psychrometric chart). “Free cooling”is exemplified by cooling efficiency so high that the cost of energy torun the system is of no consequence. For example, cooling withoutdrying/dehumidifying may be performed by coolers described herein whenthe humidity ratio is below about 80 (and the dry bulb temperatures areabove 60° F.) while cooling and drying may be required above thishumidity ratio at which point the cooler can be operated with flowingliquid desiccant. Such “free” cooling is practical for relatively largenumbers of days in less humid areas of the world (such as the southwestportion of the United States).

Embodiments of an indirect evaporative cooler according to the abovedescription and attached figures can be provided as a single unit thatprovides an integral heat and a mass transfer device utilizing a numberof separation walls. The transfer device or assembly uses membranecontainment and air flows do not come in direct contact with desiccantor water (coolant). The coolers use evaporative cooling to drive heatand mass exchange, with heat being transferred through the separationwalls between the liquid desiccant and the coolant. The heat exchange isbetween two counter and/or cross flowing air streams. The mass exchange,such as during dehumidification, is generally the transfer of watervapor from the inlet supply air or process air through a watermolecule-permeable membrane to a liquid state (e.g., to absorption bythe liquid desiccant). The evaporative section of the coolers drivesheat through the separation wall and expels that heat by evaporationfrom the coolant/water to an air stream.

It should also be kept in mind that the first and second stages(dehumidifier and indirect evaporative cooler) may be provided in asingle system or machine and be packaged to be within a single housingor two or more housings within a single system. In some cases, thefirst, second, and third (when included) stages are provided in a singlesystem with one, two, or more housings or machines. In other cases,though, the first and second stages are not packaged into the samemachine but are configured to cool the same building space. The thirdstage if present may be provided with the second stage or in a separatemachine/housing. Particularly, the dehumidifier can be packaged into amachine and take a mixture of indoor and outdoor air, dehumidify thatair, then deliver it to the space. A second machine may pull air fromthe space (and maybe some outdoor air) and send it through the indirectevaporative cooler as described, then deliver colder supply air to thespace. The second machine could exhaust some air so the dehumidifiermachine would supply some make-up air from the outside. Essentially, thesame process takes place as shown for single systems and/or machines butin two separate machines, one for each heat and mass exchanger.

In some embodiments, a method has been described for conditioning asupply air stream. The method includes the step of dehumidifying thesupply air stream to provide a dehumidified air stream. Suchdehumidifying includes directing the supply air stream through a firstchannel defined by a surface of a vapor permeable membrane containingliquid desiccant. The method further includes transferring heat from theliquid desiccant to a layer of coolant flowing in a second channeladjacent to the liquid desiccant.

In some implementations of the method, the layer of coolant comprisescoolant flowing in a layer of wicking material positioned for heattransfer from the liquid desiccant. Then, the method may further includeevaporating a portion of the coolant flowing in the layer of wickingmaterial into an air stream flowing adjacent to the layer of wickingmaterial. It may be useful in the method for the coolant in the layer tobe flowing at a flow rate of less than about 50 inches per minute. Insome cases, a temperature of the supply air stream is increased duringthe dehumidifying less than about 5° F. In the same or other cases, thedewpoint temperature of the supply air stream is decreased during thedehumidifying to less than about 55° F.

In some embodiments of the indirect evaporative cooler or supply airconditioning apparatus, a dehumidifier is included in a first stage, andthe dehumidifier performs cooling of an air stream input to an inlet ofthe dehumidifier by absorption of water vapor from the input air stream.Such cooling may be performed using a membrane-contained liquiddesiccant that is liquid cooled. The phrase “liquid cooled” may take anumber of meanings including, but not limited to, flowing a liquidcoolant such as water adjacent the channel containing the liquiddesiccant, and the liquid coolant flows such that the temperature of theliquid is raised as it passes through the dehumidifier to carry awayheat from the liquid desiccant.

The apparatus may also include a second stage comprising an indirectevaporative cooler in fluid communication with an outlet of the firststage to receive a cooled portion of the input air stream. The indirectevaporative cooler may be operable to sensibly cool the received andcooled portion of the input air stream to a temperature (or to be withina temperature range) less than the wet bulb temperature and greater thanthe dew point temperature of the input air stream. During operation ofthe apparatus, a first portion of the sensibly cooled air stream issupplied to a cooled space and a second portion of the sensibly cooledair stream is directed to a wet side of the indirect evaporative coolerand receives heat from the received and cooled portion of the input airstream.

In some implementations of such an apparatus, the dehumidifier performsthe cooling using a membrane-contained liquid desiccant cooled by anindirect evaporative channel that includes a wicking layer containing aflow of coolant. In other implementations, the dehumidifier performs thecooling using a membrane-contained liquid desiccant that is liquidcooled. The apparatus may be operated such that the second portioncomprises 10 to 40 percent of the received and cooled portion of theinput air. It may be useful for a flow channel for the second portion ofthe sensibly cooled air stream in the indirect evaporative cooler to bedefined in part by a surface of a separation wall. Then, a layer ofwicking material may be provided on the surface acting to wick a streamof liquid coolant adjacent the flow channel.

In some settings, the apparatus includes a direct evaporative stagebetween the second stage and the cooled space. Then, during operations,the direct evaporative stage receives the first portion of the sensiblycooled air stream and provides additional cooling via direct evaporativecooling prior to supplying the first portion of the sensibly cooled airstream to the cooled space. Further, the direct evaporative stage may beadapted to contain flow of water with a vapor permeable membrane orwithin a layer of wicking material (e.g., the layer of wicking materialis provided as a surface with a hydrophilic coating).

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions, and sub-combinations thereof. It is thereforeintended that the following appended claims and claims hereafterintroduced are interpreted to include modifications, permutations,additions, and sub-combinations to the exemplary aspects and embodimentsdiscussed above as are within their true spirit and scope.

1. An apparatus for dehumidifying air supplied to a space, comprising: afirst flow channel for receiving a stream of inlet supply air, whereinthe first flow channel is defined in part by a vapor permeable membrane;a second flow channel adjacent to the first flow channel receiving astream of liquid desiccant; and a third flow channel adjacent to thesecond flow channel receiving a stream of exhaust air, whereby heat istransferred from the stream of inlet supply air to the stream of exhaustair in the third flow channel via the stream of liquid desiccant,wherein the third flow channel is defined in part by a first surface ofa separation wall, wherein the separation wall is spaced apart adistance from the vapor permeable membrane, and wherein a layer ofwicking material is provided on the first surface acting to wick astream of liquid coolant.
 2. The apparatus of claim 1, wherein thesecond flow channel is defined by the vapor permeable membrane and asecond surface of a separation wall, opposite the first surface, that isimpermeable to fluid.
 3. The apparatus of claim 1, wherein the stream ofliquid coolant flow through the wicking material layer is gravitydriven.
 4. The apparatus of claim 1, wherein the wicking material isknitted nylon fabric, polypropylene woven fabric, polypropylenenon-woven fabric, adhesive-backed flocking fibers, or one or morefabrics coated with a hydrophilic coating and wherein the liquiddesiccant comprises a salt solution and the liquid coolant compriseswater.
 5. The apparatus of claim 1, wherein the layer of wickingmaterial comprises a hydrophilic coating on the first surface of theseparation wall.
 6. The apparatus of claim 1, wherein the exhaust aircomprises a portion of the stream of inlet supply air exiting the firstflow channel at about the second, lower enthalpy.
 7. The apparatus ofclaim 1, wherein the stream of inlet supply air flows in a firstdirection in the first flow channel and the stream of exhaust air flowsin a second direction in the third flow channel, the second directionbeing in at least one of cross or counter to the first direction, andwherein the third flow channel is arranged such that the exhaust airstream flows in an at least a partially cross or counter flow directionrelative to the stream of inlet supply air in the first flow channel. 8.A method of conditioning a supply air stream, comprising: dehumidifyingthe supply air stream to provide a dehumidified air stream, wherein thedehumidifying includes directing the supply air stream through a firstchannel defined by a surface of a vapor permeable membrane containingliquid desiccant; and transferring heat from the liquid desiccant to alayer of coolant flowing in a second channel adjacent to the liquiddesiccant.
 9. The method of claim 8, wherein the layer of coolantcomprises coolant flowing in a layer of wicking material positioned forheat transfer from the liquid desiccant and wherein the method furtherincludes evaporating a portion of the coolant flowing in the layer ofwicking material into an air stream flowing adjacent to the layer ofwicking material.
 10. The method of claim 9, wherein the coolant in thelayer is flowing at a flow rate of less than about 50 inches per minute.11. The method of claim 8, wherein a temperature of the supply airstream is increased during the dehumidifying less than about 5° F. 12.The method of claim 8, wherein the dewpoint temperature of the supplyair stream is decreased during the dehumidifying to less than about 55°F.
 13. An apparatus for conditioning an inlet air stream, comprising: afirst stage comprising a dehumidifier operable for cooling an air streaminput to an inlet of the dehumidifier by absorption of water vapor fromthe input air stream; and a second stage comprising an indirectevaporative cooler in fluid communication with an outlet of the firststage to receive a cooled portion of the input air stream, wherein theindirect evaporative cooler is operable to sensibly cool the receivedand cooled portion of the input air stream to a temperature range lessthan the wet bulb temperature and greater than the dew point temperatureof the input air stream; wherein a first portion of the sensibly cooledair stream is supplied to a cooled space, and wherein a second portionof the sensibly cooled air stream is directed to a wet side of theindirect evaporative cooler and receives heat from the received andcooled portion of the input air stream.
 14. The apparatus of claim 13,wherein the dehumidifier performs the cooling using a membrane-containedliquid desiccant cooled by an indirect evaporative channel that includesa wicking layer containing a flow of coolant.
 15. The apparatus of claim13, wherein the dehumidifier performs the cooling using amembrane-contained liquid desiccant that is liquid cooled.
 16. Theapparatus of claim 13, wherein the second portion comprises 10 to 40percent of the received and cooled portion of the input air.
 17. Theapparatus of claim 13, wherein a flow channel for the second portion ofthe sensibly cooled air stream in the indirect evaporative cooler isdefined in part by a surface of a separation wall and wherein a layer ofwicking material is provided on the surface acting to wick a stream ofliquid coolant adjacent the flow channel.
 18. The apparatus of claim 13,further comprising a direct evaporative stage between the second stageand the cooled space, wherein the direct evaporative stage receives thefirst portion of the sensibly cooled air stream and provides additionalcooling via direct evaporative cooling prior to supplying the firstportion of the sensibly cooled air stream to the cooled space.
 19. Theapparatus of claim 18, wherein the direct evaporative stage is adaptedto contain flow of water with a vapor permeable membrane or within alayer of wicking material.
 20. The apparatus of claim 19, wherein thelayer of wicking material is provided as a surface with a hydrophiliccoating.
 21. A method for conditioning a supply air stream, comprising:cooling an air stream by absorption of water vapor from the air streamto a first wet bulb temperature and first dewpoint temperature; afterthe cooling by absorption, using indirect evaporative cooling tosensibly cool the air stream to a second temperature; and supplying afirst portion of the sensibly cooled air stream to a space, wherein asecond portion of the sensibly cooled air stream is used to perform theindirect evaporative cooling of the air stream.
 22. The method of claim21, wherein the second temperature is less than the first wet bulbtemperature and greater than the first dewpoint temperature and thecooling is performed using membrane-contained liquid desiccant or with aliquid coolant flowing on a separation wall covered with a layer ofwicking material or a hydrophilic coating.
 23. The method of claim 21,further comprising, prior to the supplying the first portion to thespace, humidifying the sensibly cooled air stream by providing a flow ofwater adjacent to the sensibly cooled air stream, wherein the flow ofwater with a vapor permeable membrane or within a layer of wickingmaterial.
 24. The method of claim 21, further comprising, prior to thesupplying the first portion to the space, providing additional coolingto the sensibly cooled air stream via direct evaporative cooling.